1. Field of the Invention
The present invention relates in general to filtering an audio broadcast signal, and more specifically, to a method for switching bandwidths in the presence of a duplicative signal.
2. Description of the Related Art
With the introduction of terrestrial digital audio broadcasting, a radio listener has the opportunity to experience a higher quality of broadcast reception utilizing digital broadcast as opposed to the traditional analog broadcast. Digital audio broadcasting offers improved reception, better audio quality, and enhanced data services.
A duplicative audio broadcast system simultaneously transmits 1) a primary channel having at least a main program content and preferably including a supplemental data stream, and 2) a backup channel with main program content that is at least a partial duplicate of the main program content in the primary channel but usually without the supplemental data stream. Certain differences in the backup channel transmission allow it to be received by a receiver under conditions in which the primary channel is unreceivable.
To receive the full benefits of digital audio broadcasting, listeners must have a digital audio receiver to receive the digital audio broadcast. Therefore there will be a transition period as listeners migrate from analog audio receivers to digital audio receivers. During the transition period, if a broadcast station wants to maintain their listening audience, it must transmit both a digital broadcast for those listeners who are capable of receiving the digital broadcast and an analog broadcast for those listeners who are still utilizing the analog audio receiver.
A method of simulcasting both the digital signal and the analog signal over a frequency range for a particular audio broadcast station has recently become available. In-Band On Channel (IBOC) is a system that has been adapted to transmit a hybrid signal comprising an analog signal on a center portion of the frequency range and a digital signal occupying an upper and lower side portion of the frequency range. Digital radio receivers that have been adapted to receive both the digital signal and the analog signal uses the digital signal as its primary channel and the analog signal as a backup channel.
A preferred mode of reception is to receive and reproduce the digital signal as opposed to the analog signal due to the enhanced sound quality and the supplemental data stream provided by the digital signal. However, due to the differences in the transmission power levels, propagation, and performance in fringe areas, the coverage area of the analog signal in which a useful signal can be received is typically larger than the coverage area for the digital signal. A hybrid receiver needs to know if a digital signal is present, and if so, whether the signal quality is strong enough to receive the digital signal. If the digital signal is not present or the signal quality is below a certain threshold, then the backup channel must be processed to receive the audio broadcast.
Eventually, hybrid stations may switchover to an all digital audio broadcast station while retaining the duplicative nature of the broadcast signal (i.e., both the primary and backup channels are digital). In the all digital duplicative system, the backup channel transmits at a lower effective data rate in order to provide a more robust signal that can be received during times that (or at a place where) the primary channel is impaired. In order to achieve a lower data rate, the supplemental data and possibly some portion of the main program content are omitted from the backup channel (e.g. by encoding the main audio program at a lower bit rate).
Each broadcast signal is adapted to occupy its assigned frequency channel wherein the broadcast signal contains a transmitted energy used to carry information such as a main audio program or other related broadcast data. Energy from interfering channels (e.g., adjacent channels) may still be present in the channel of interest and may impede the reception of the desired broadcast signal. It is known to adapt the receiver bandwidth to the signal conditions in order to improve signal-to-noise ratio (SNR) of a reproduced audio signal and eliminate unwanted energy transmitted from the adjacent channels.
Since the IBOC system transmits the primary signal in the upper and lower sideband portions the frequency channel, a wide bandwidth as opposed to a narrow bandwidth must be used to pass the primary signal. However, since it is undetermined whether a primary signal is present when initially tuning to the target frequency channel, the audio output may include adjacent channel interference or other noise or distortion if the passband is set to the wide bandwidth and the primary signal is not present.
Under weak signal conditions (e.g., fringe areas), the receiver may conclude that a station is analog (backup) only and therefore adopt a narrow bandwidth even though the station is a hybrid station. When the digital (primary) signal gets stronger (e.g., vehicle closer to the transmitter), the receiver may not detect the primary signal because the receiver is in narrow bandwidth. Therefore, a lower quality backup signal may continue to be used even after a usable primary signal is present.